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Author Topic: Practical recrystallization question  (Read 2932 times)

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SugarSkull

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Practical recrystallization question
« on: May 07, 2011, 08:00:13 AM »

This might be a dumb question, but I would appreciate any advice from other members.
Any tips for when your crystals are so fine they go through the filter and end up in the filtrate? The only 'solution' (parden the pun) I could think of was to take off the solvent directly on the rotovap, but naturally am worried about my crystals dissolving again and coming off on the rotovap too. My recryst solvent was ethanol.

Cheers!
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Honclbrif

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Re: Practical recrystallization question
« Reply #1 on: May 07, 2011, 09:22:17 AM »

The reason you filter your crystals is because the solvent carries away the soluble impurities. If you vac off the solvent, you won't have accomplished anything at all.

If you do know of something that your crystals are soluble in, you could filter through celite, then redissolve your crystals after the impurities have been removed. With repeated rounds of recrystallization as the impurities decrease, you might start to get crystals which are large enough to filter with standard paper. Alternatively, if you have access to a centrifuge, you can always spin them down and decant.
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enahs

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Re: Practical recrystallization question
« Reply #2 on: May 07, 2011, 12:41:47 PM »

Get a finer filter? Smaller filter pores.
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asa029

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Re: Practical recrystallization question
« Reply #3 on: May 07, 2011, 09:16:08 PM »

You could also just remove the solvent using a normal pasteur-pipette. This is done by carefully pushing the end of the pipette against the wall of the container and carefully pulling in the solvent. It takes some skill, but usually most of the solvent can be removed this way. THis method is only applicable to micro and half-macro scale experiments. If you are doing macro, a finer filter-paper or just a better recrystallization is needed.

Small crystals are often a sign that the recrystallization hasn't been done properly (not always though!). Are you sure that you have used a correct solvent and a proper temperature gradient?
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Honclbrif

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Re: Practical recrystallization question
« Reply #4 on: May 09, 2011, 08:46:14 AM »

You could also just remove the solvent using a normal pasteur-pipette.

If the crystals are too small for filter paper, I doubt a pipette filtration will catch them. I've always found that the finest filter you can get is a good celite pad.
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Åke

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Re: Practical recrystallization question
« Reply #5 on: May 23, 2011, 01:02:53 AM »

Try Munktell OOH grade filter paper, alternatively a Büchner funnel with a sintered glass disc of porosity # 4.
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cpncoop

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Re: Practical recrystallization question
« Reply #6 on: May 26, 2011, 04:13:07 AM »

Try heat cycling to increase the crystal size. The idea is you reheat until almost everything is dissolved, and the remaining solids serve as nucleation sites for crystal growth, hence giving bigger crystals.
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